1. Field of the Invention
The present invention relates to a focusing plate used in a single-lens reflex camera, and particularly to a focusing plate in which a phase grating is formed by rotating molecules of a liquid crystal by applying an electrical field to form a distribution of refractive indexes in a liquid crystal layer.
2. Description of the Prior Art
Hitherto, it has been previously known that a liquid crystal, particularly a scattering liquid crystal which is operated in a dynamic scattering mode, may be used in the focusing plate of a camera.
FIG. 1 is a sectional view of the configuration of a conventional scattering liquid crystal cell in which transparent electrodes 13, 14 made of thin films that consist of, for example, indium oxide and tin oxide are vapor-deposited over the entire inside surfaces of glass base plates 11, 12, respectively, and in which a liquid crystal layer 16 is sealed in a frame 15 held between the electrodes 13, 14. The two transparent electrodes 13, 14, are connected to a voltage power source 17 so that an electrical field may be applied to the liquid crystal layer 6 and a switch 18.
FIG. 2 shows the change that occurs in the vertical transmittance of white light L vertically incident upon the liquid crystal cell shown in FIG. 1 comprising a scattering liquid crystal with changes in the applied voltage.
Examples of conventional means for changing the scattering characteristics of a focusing plate using a liquid crystal cell include the method disclosed in Japanese Patent Publication No. 37379/1973 in which the whole of a focusing plate is changed from a transparent state to a diffused state by applying a voltage, and the method disclosed in Japanese Patent Laid-Open No. 15523/1975 in which the diffusing property is changed by changing the voltage to be applied.
The aforementioned scattering liquid crystal produces the effect of scattering light due to molecular groups of the liquid crystal in a turbulent state in the liquid crystal layer. The scattering liquid crystal therefore has a disadvantage in that, if the molecular groups of the liquid crystal producing such scattering effect are arranged in a finder with a magnification of several times such as the finder of a camera, the scattering can be seen by the naked eye even though the molecular groups have sizes of as small as several .mu.m to several hundreds .mu.m, Thus, a focusing plate is low in quality and scattering liquid crystal has not yet been put into practical use in a focusing plate.